To keep up with the trend in the ionics field toward downsizing and entirely solid formation, attempts are being aggressively made to apply an entirely solid primary battery, secondary battery or electric double layer capacitor using a solid electrolyte to electrochemical elements, as a new ion conductor which can take the place of conventional electrolytic solutions.
More specifically, electrochemical elements using a conventional electrolytic solution are deficient in the long-term reliability because liquid leakage outside the component or elution of the electrode active material is readily caused. However, products using a solid electrolyte are free of such a problem and can be easily formed to have a small thickness. Furthermore, the solid electrolyte has excellent heat resistance and is advantageous also in the manufacturing process of a product such as a battery.
In particular, when a solid polymer electrolyte mainly comprising a polymer compound is used, the battery can have increased flexibility as compared with those using an inorganic material and accordingly, can be formed into various shapes. However, batteries heretofore investigated have a problem that due to low ion conductivity of the solid polymer electrolyte, the takeout current is small.
In recent years, many studies have been made on lithium secondary batteries using a metal oxide or metal sulfide such as LiCoO.sub.2, LiNiO.sub.2, LiMnO.sub.2 and MoS.sub.2 for the positive electrode, and lithium, lithium alloy, a carbon material, inorganic compound or polymer compound capable of occluding and releasing lithium ion for the negative electrode. For example, J. of Electrochem. Soc., vol. 138 (No. 3), page 665 (1991) reports a battery using MnO.sub.2 or NiO.sub.2 for the positive electrode. This is high in the capacity per weight or per volume and drawing attention.
Further, an electric double layer capacitor comprising polarizable electrodes formed of a carbon material having a large specific area, such as activated carbon or carbon black, having disposed therebetween an ion conductive solution is often used in these days as a memory backup power source. For example, Kino Zairyo (Functional Materials), page 33, (February, 1989) describes a capacitor using carbon-base polarizable electrodes and an organic electrolytic solution; and 13th Electrochemical Society Meeting Atlanta Ga., No. 18 (May, 1988) describes an electric double layer capacitor using an aqueous sulfuric acid solution.
Further, JP-A-63-244570 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a capacitor using Rb.sub.2 Cu.sub.3 I.sub.3 Cl.sub.7 having high electrical conductivity as an inorganic solid electrolyte.
However, the battery or electric double layer capacitor using an existing electrolytic solution is deficient in the long-term use or reliability because liquid leakage outside the battery or capacitor is readily caused under an abnormal condition such as when the battery or capacitor is used for a long period of time or a high voltage is applied. On the other hand, the battery or electric double layer capacitor using a conventional inorganic ion conductive material has a problem that the ion conductive material is low in the decomposition voltage and accordingly, the output voltage is low, or has a problem in the production process because an interface between the electrolyte and the electrode is difficult to form.
JP-A-4-253771 proposes to use a polyphosphazene-base polymer compound as an ion conductive material of a battery or electric double layer capacitor. The battery or electric double layer capacitor using a solid ion conductive material mainly comprising such a polymer compound is advantageous in that the output voltage is high as compared with those using an inorganic ion conductive material and it can be formed into various shapes and easily sealed. However, in this case, the ion conductivity of the solid polymer electrolyte is not sufficiently high and it is approximately from 10.sup.-4 to 10.sup.-6 S/cm, as a result, the takeout current is disadvantageously small. Furthermore, in assembling a solid electrolyte together with polarizable electrodes in a capacitor, it is difficult to uniformly compound the solid electrolyte with the carbon material having a large specific area because the materials mixed are both a solid.
The solid polymer electrolytes under general investigations are improved in the ion conductivity up to approximately from 10.sup.-4 to 10.sup.-5 S/cm at room temperature, however, this still stays in a level lower by two figures than that of the liquid ion conductive material. Further, at low temperatures of 0.degree. C. or less, the ion conductivity generally lowers to an extreme extent. Furthermore, when the solid electrolyte is compounded and assembled with an electrode of an element such as a battery or electric double layer capacitor or when the solid electrolyte is formed into a thin film and assembled in an element such as a battery or electric double layer capacitor, difficult techniques are necessary for the working of compounding or continuously contacting the solid polymer electrolyte with an electrode, thus, a problem is also present in the process of producing an element.
As an example of the solid polymer electrolyte, Br. Polym. J., Vol. 319, page 137 (1975) reports that a composite material of a polyethylene oxide with an inorganic alkali metal salt exhibits ion conductivity, but the ion conductivity thereof at room temperature is as low as 10.sup.-7 S/cm.
Many reports have been issued in recent years stating that a comb-type polymer having introduced into the side chain thereof oligooxyethylene is intensified in the thermal mobility of oxyethylene chain which undertakes the ion conductivity, and thereby improved in the ion conductivity.
For example, J. Phys. Chem., Vol. 89, page 987 (1984) describes an example where oligooxyethylene is added to the side chain of polymethacrylic acid and an alkali metal salt is compounded thereto. Further, J. Am. Chem. Soc., Vol. 106, page 6854 (1984) describes polyphosphazene having an oligooxyethylene side chain, compounded with an alkali metal salt, however, the ion conductivity is about 10.sup.-5 S/cm and still insufficient.
U.S. Pat. No. 4,357,401 reports that a solid polymer electrolyte comprising a salt ionizable with a cross-linked polymer having a hetero atom is reduced in the crystallinity, has a low glass transition point and is improved in the ion conductivity, but the ion conductivity is about 10.sup.-5 S/cm and still insufficient.
J. Appl. Electrochem., Vol. 5, pp. 63-69 (1975) reports that a so-called polymer gel electrolyte obtained by adding a solvent and an electrolyte to a cross-linked polymer compound, such as polyacrylonitrile or polyvinylidene fluoride gel, has a high ion conductivity. Further, JP-B-58-36828 (the term "JP-B" as used herein means an "examined Japanese patent publication") reports that a polymer compound gel electrolyte similarly obtained by adding a solvent and an electrolyte to a polymethacrylic acid alkyl ester has a high ion conductivity.
However, despite the high ion conductivity, these polymer gel electrolytes are disadvantageous in that due to the fluidity imparted, they cannot be handled as a complete solid, have poor film strength or film formability, readily cause short circuit when applied to an electric double layer capacitor or a battery, and have a problem in view of sealing similar to the case of using a liquid ion conductive material.
U.S. Pat. No. 4,792,504 proposes to improve the ion conductivity by using a cross-linked solid polymer electrolyte where the continuous network of polyethylene oxide is impregnated with an electrolytic solution comprising a metal salt and an aprotic solvent. However, the ion conductivity is 10.sup.-4 S/cm and still insufficient. Further, as a result of addition of a solvent, a problem arises that the film strength is reduced.
JP-B-3-73081 and its corresponding U.S. Patent (U.S. Pat. No. 4,908,283) disclose a process for forming a solid polymer electrolyte by irradiating an active ray such as ultraviolet ray, on a composition comprising an acryloyl-modified polyalkylene oxide such as polyethylene glycol diacrylate/an electrolyte salt/an organic solvent, thereby reducing the polymerization time.
Also, U.S. Pat. Nos. 4,830,939 and 5,037,712 disclose a process for forming a solid polymer electrolyte containing an electrolytic solution similarly by irradiating radiation such as ultraviolet ray or electron beam, on a composition comprising a cross-linking polyethylenic unsaturated compound/an electrolyte salt/a solvent inactive to active ray. In these systems, the ion conductivity is improved due to the increase of the electrolytic solution in the solid polymer electrolyte, however, it is still insufficient and the film strength is liable to decrease.
U.S. Pat. No. 5,609,974 discloses a solid polymer electrolyte using across-linked polymer compound having introduced thereinto a monocarbonate side chain so as to elevate the dissociation ability of the electrolyte salt, however, the amount of carbonate introduced is small and satisfactory capabilities such as ion conductivity and current characteristics cannot be obtained.
U.S. Pat. No. 5,001,023 describes an electrochemical device using as a component of solid polymer electrolyte a polymer to which a side chain having no active hydrogen atoms is bonded and mentions poly(ethylene-ether carbonate) with methacrylate terminal cap as an example of the said polymer. However, these polymers are insufficient in ion conductivity and cannot be polymerized when containing much solvent due to poor polymerizability of methacrylate, and have a problem in workability as a solid polymer electrolyte gel.
JP-A-9-147912 describes a solid polymer electrolyte which has flexibility as well as rigidity and is improved in affinity to metal electrodes and interfacial resistance by using a copolymer of poly(alkylene(ether) carbonate) with methacrylate terminal cap similar to that described in U.S. Pat. No. 5,001,023 and polyether with methacrylate terminal cap. However, they have a problem in durability of polyether chains and also have problems that the polymerization reaction unevenly proceeds because of the use of two or more methacrylates and that not a few double bonds remain since the polymerizability of methacrylate is not sufficient, which also causes a deficiency in durability.
JP-A-8-295715 proposes a solid polymer electrolyte which has improved ion conductivity and film-forming properties and exhibits less polymerization shrinking by using urethane acrylate having particular structures comprising polyether or polyester units. However, these compounds are synthesized through a reaction between a compound having hydroxyl groups at both its ends and a diisocyanate and contain many by-products. Accordingly, they have a problem in the stability of ion conductivity and electrochemical properties. Furthermore, there also remains a problem in durability since they contain polyether and/or polyester. This publication also mentions other structures in which polycarbonate is introduced in place of the polyether or the like but it does not give sufficient consideration to such properties like affinity to the electrodes, for the mentioned structures including those containing aromatic rings both in polycarbonate and diisocyanate, for instance. Further, this publication does not give any specific examples of the production of the solid polymer electrolyte with the use of a polymer containing polycarbonate chains and does not investigate the properties of such electrolyte.
Solid State Ionics, No. 7, page 75 (1982) reports that by further compounding an alumina particle to a LiClO.sub.4 /polyethylene oxide composite material as a solid polymer electrolyte, the strength of the solid polymer electrolyte can be improved without reducing the ion conductivity. WO94/06165 proposes a solid electrolyte comprising a polyalkylene oxide/isocyanate cross-linked entity/inorganic oxide composite material impregnated with a non-aqueous electrolytic solution, with the intention of increasing strength of the solid polymer electrolyte containing an electrolytic solution. However, these composite solid polymer electrolytes are not satisfied in the characteristics of the polymer compound itself and their practical use has a problem in view of the ion conductivity, workability and stability.
JP-A-62-272161 discloses an electric double layer capacitor comprising a combination of a solid polymer electrolyte using a polymer compound such as cyanoethyl cellulose with an activated carbon electrode, however, the solid polymer electrolyte used is insufficient in the ion conductivity and difficult to compound with the activated carbon electrode, and an electric double layer capacitor having satisfactory capabilities has not yet been obtained.
In order to solve these problems, the present inventors have proposed an ion conductive solid polymer electrolyte using a composite material comprising a polymer obtained from a (meth)acrylate prepolymer having a urethane bond and containing an oxyalkylene group, and an electrolyte salt (JP-A-6-187822). This solid polymer electrolyte has an ion conductivity of a high level such that in the state where a solvent is not added, the ion conductivity is 10.sup.-4 S/cm (at room temperature) and when a solvent is added, it increases to 10.sup.-3 S/cm or more even at room temperature or a temperature lower than that. Further, the film quality is good and improved to such a degree that the electrolyte can be obtained as a self-standing film. Furthermore, the prepolymer has good polymerizability and is advantageous in that when applied to a battery, it can be integrated into a battery in the state of a prepolymer and then polymerized into a solid.
However, this system has also a problem that the film strength is deficient for the use as a separator of a battery or the like and handling in an industrial scale is difficult. Further, the polymer, particularly the oxyalkylene moiety at a high temperature, is readily deteriorated due to a slight amount of impurities within the battery system, such as water content, decomposition product of the electrolyte salt or electrode material impurity, and this adversely affects the life of the battery.
Furthermore, there is a problem that when the system is applied to a battery or an electric double layer capacitor, the capacity is greatly reduced at the discharging of a large current. This is considered to occur because the dielectric constant of the polymer compound is still insufficient and accordingly, the electrolyte salt cannot dissociate or move satisfactorily within the solid polymer electrolyte.